1. Field of the Invention
The invention relates to projection systems for photolithography, and particularly to catadioptric systems including first and second optical imaging groups for 157 nm lithography.
2. Discussion of the Related Art
Extending DUV lithography to sub 100-nm linewidths requires a projection system with a high numerical aperture, e.g., 0.65-0.75 or larger, at a wavelength of 157 nm. As optical lithography is extended into the vacuum ultraviolet (VUV), issues surrounding the laser linewidth and material availability could cause substantive delays to the development of a commercial 157 nm step/repeat or step/scan tool. Therefore, it is desired to investigate optical configurations that minimize the consumption of calcium fluoride.
Microlithographic reduction projection catadioptric objectives, such as that shown and described with respect to FIG. 3 of European patent application no. EP 0 779 528 A2, which is hereby incorporated by reference, may be understood as variants of pure catoptric objectives. FIG. 3 of the '528 application shows a system having six minors and three lenses. The optical surfaces are generally symmetric to a common axis, and the object plane and the image plane are situated on this same axis upstream and downstream of the objective, respectively. As described in the '528 application, the system of FIG. 2 therein has a numerical aperture of only 0.55 and that of FIG. 3 therein only 0.6. In addition, all but one of the six mirrors shown at FIG. 3 are cut off sections of a bodies of revolution, yielding mounting and adjustment face difficulties. Also, the lenses shown in FIG. 3 serve only as correcting elements having minor effect. In addition, the most imageward (or optically closest to the image plane) mirror described in the '528 application is concave. It is desired to have an objective with a higher numerical aperture, and which is constructed for easier mounting and adjustment.
A similar objective to that described in the '528 application (above) is disclosed at U.S. Pat. No. 4,701,035, which is hereby incorporated by reference. The objective shown at FIG. 12 of the '035 patent, for example, has nine mirrors, two lenses and two intermediate images. The object plane and image plane are situated within the envelope of the objective. The objective described in the '035 application also exhibits a low numerical aperture and offers similar mounting and adjustment difficulties as described above with respect to the '528 application. In both the '528 and '035 applications, the image field is an off-axis ring sector.
An axially symmetric type of catadioptric objective is disclosed in German patent document DE 196 39 586 A (see also U.S. patent application Ser. No. 09/263,788), each application of which is hereby incorporated by reference. The '586 application discloses an objective having two opposing concave mirrors, an image field centered at the common axis and a central obscuration of the aperture. It is recognized herein that it is desired to have an axially symmetric objective having an unobscured aperture. Another type of catadioptric objective for microlithographic reduction projection has only one concave mirror and a folding mirror, as is described at U.S. Pat. No. 5,052,763 and European patent application no. EP 0 869 383 A, which are each hereby incorporated by reference.
It is recognized herein that catadioptric optical systems have several advantages, especially in a step and scan configuration, and that it is desired to develop such systems for wavelengths below 365 nm. One catadioptric system concept relates to a Dyson-type arrangement used in conjunction with a beam splitter to provide ray clearance and unfold the system to provide for parallel scanning (see, e.g., U.S. Pat. Nos. 5,537,260, 5,742,436 and 5,805,357, which are incorporated by reference). However, these systems have a serious drawback since the size of this beam splitting element becomes quite large as the numerical aperture is increased up to and beyond 0.65 to 0.70, making the procurement of bulk optical material with sufficient quality (in three-dimensions) a high risk endeavor. This problem is exacerbated as wavelengths are driven below 193 nm because the selection of material that can be manufactured to lithographic quality is severely limited.
To circumvent this problem, it is recognized herein that it is desired to develop systems without beamsplitters. However, it is difficult to achieve an adequately high numerical aperture (e.g., U.S. Pat. Nos. 4,685,777, 5,323,263, 5,515,207 and 5,815,310, which are incorporated by reference), or to achieve a fully coaxial configuration, instead of relying on the use of folding mirrors to achieve parallel scanning (e.g., U.S. Pat. No. 5,835,275 and EP 0 816 892, which are incorporated by reference) and thereby complicating the alignment and structural dynamics of the system. In addition, it is desired to have an optical design that generally does not utilize too many lens elements, which can greatly increase the mass of the optical system.
WO 01/51 979 A (U.S. Ser. Nos. 60/176,190 and 09/761,562) and WO 01/55767 A (U.S. Ser. Nos. 60/176,190 and 09/759,806)—all commonly owned and published after the priority date of this application—show similar coaxial catadioptric objectives with 4 mirrors or more.
EP 1 069 448 A1 published after the priority date of this application shows a coaxial catadioptric objective with two curved mirrors and a real. intermediate image located besides the primary mirror.
All cited publications are incorporated herein by reference in their entirety. It is desired to develop a compact, coaxial, catadioptric projection system for deep ultraviolet and/or vacuum ultraviolet lithography that uses no beamsplitters or fold mirrors in is optical path.
It is an object of the invention to provide an objective for microlithographic projection reduction having high chromatic correction of typical bandwidths of excimer laser light sources, which permits a high image-side numerical aperture, and which reduces complexity with respect to mounting and adjusting.